The present invention relates to a friction vacuum pump comprising at least one turbomolecular pump stage, with a molecular pump stage that is subsequently connected on the pressure side, and with a transition stage mounted between the turbomolecular pump stage and the molecular pump stage.
In turbomolecular pumps with molecular pump stages, commonly designed as screw pump stages, subsequently connected on the pressure side, also called compound pumps, there exists the problem of a changing flow characteristic in the pumped gases in the transition area from molecular (at pressures below 10−3 mbar) to laminar (from about 10−2 mbar upwards). As the pumped gas changes from the turbomolecular stage into the screw stage, the pumped gas needs to be deflected from a primarily tangential direction of flow to a primarily axial direction of flow. The radial dimension of the flow channel is tapered considerably. Across a very short distance, a large change in the axial cross section of the pump chamber needs to be implemented. Known embodiments of this transition area have the disadvantage of incurring losses in the flow. These impair to a considerable extent the pumping capacity of the pump.
From DE 297 17 079 a friction vacuum pump having the aforementioned characteristics is known. Part of the transition stage is a centrifugal pump formed by ridges on the rotor side extending substantially in the radial direction. This solution does in fact have the effect of deflecting the gases into the screw stage; however, its pumping effect is limited. Moreover, the known solution requires that the diameter of the screw pump stage be greater than the diameter of the turbomolecular stage. For this reason it is not usable in high pumping capacity friction pumps, since the diameter of the rotor in the molecular pump stage is subject to restrictions in size owing to the high centrifugal forces. Finally as to the arrangement of the ridges on the rotor side it holds that their manufacture is involved and that they are not uncritical as to material tensions.
The content of U.S. Pat. No. 6,168,374, moreover, belongs to the state-of-the-art. From this it is known to provide between the turbomolecular pump stage and the molecular pump stage that is subsequently connected, a filling stage which is equipped with wings. Also this solution is difficult to manufacture. Moreover, there result during operation, high mechanical tensions in the area of the wings bases.
It is the task of the present invention to create a vacuum engineering wise optimised transition of the turbomolecular range to the molecular range without suffering the disadvantages detailed.